The present invention relates to improvements in internal combustion engines. More particularly, the invention is concerned with an improved position control means enabling the occurrence of dead-centers to be controlled in an internal combustion engine.
In conventional internal combustion engines, the crankshaft is usually arranged below a cylinder in a manner such that the longitudinal axis of the cylinder and the crankshaft axis lie in a common plane. Thus, when a vertically reciprocating piston which is connected to a crank-arm of the crankshaft by means of a connecting rod reaches its uppermost position, a dead-center occurs, that is, a line passing through the points of connection of the rod to the piston and crank-arm intersects the crankshaft axis. It has therefore been the practice to time the ignition so that combustion takes place when the crank-arm has moved past dead-center and is at an angular position relative to the longitudinal axis of the cylinder. As a result, there is a loss of compression in the combustion chamber defined in the cylinder such that upon combustion maximum thrust cannot be imparted to the piston. In addition, since the connecting rod is angularly inclined relative to the longitudinal axis of the cylinder when the piston is intermediate its uppermost and lowermost positions, there is a problem of friction occurring between the piston and the cylinder wall.
In U.S. Pat. No. 4,505,239 issued Mar. 19, 1985, Applicant has proposed to overcome the aforementioned drawbacks by providing a pair of rotatably mounted parallel crankshafts arranged equidistantly relative to the longitudinal axis of the cylinder and each having a crank-arm rotatable about its respective crankshaft axis, and means coupling the crankshafts together for synchronising the rotation thereof with the crank-arm of one of the crankshafts having an angular advance over the crank-arm of the other crankshaft. A rocker member is pivotally connected to the piston for rocking movement about a pivot axis extending normal to the longitudinal axis of the cylinder. A pair of connecting rods interconnect the rocker member and the respective crank-arms of the crankshafts with each connecting rod being pivotally connected to the rocker member. The rocker member together with the connecting rods define a position control means for enabling the rocker member to rock when the piston reaches the first limit position and to thereby maintain the piston substantially stationary at the first limit position for a period of time sufficient to permit the crank-arm of the aforesaid one crankshaft to move past dead-center.
Due to the provision of two crankshafts and two connecting rods indirectly connecting the crank-arms of the crankshafts to the piston via a rocker member, and by giving to the crank-arm of one crankshaft an angular advance over the crank-arm of the other crankshaft, it is possible to control the occurrence of dead-centers so as to achieve maximum compression when the piston is at the first limit position while the crank-arm which is given an angular advance is positioned past dead-center. Since the crankshafts are disposed equidistantly relative to the longitudinal axis of the cylinder, there is also substantially no friction exerted between the piston and the cylinder wall.
The rocker member used according to Applicant's above-mentioned patent comprises a pair of fixed arms extending radially from the pivot axis about which the rocker member rocks, the arms being angularly inclined relative to one another and each being pivotally connected at a free end thereof to a respective one of the connecting rods. The arms of the rocker member have different radial lengths and the connecting rods which are pivotally connected at their upper ends to these arms and at their lower ends to the respective crank-arms of the crankshafts are arranged so as to cross each other. Due to such an arrangement, the connecting rod which is connected to the shorter arm of the rocker member must be curved adjacent its upper end to provide a clearance for the longer arm of the rocker member during the rocking movement thereof. As a result, there is a structural weakness at the curved portion of the connecting rod, which must be compensated for by increasing the thickness of the rod at that location. Such a connecting rod therefore requires special designing and tooling, which add to the manufacturing cost of the engine.
The rocker member with radially extending arms is also bulky and thus only limited space is provided inside the piston within which the rocker member may rock. Moreover, since the arms of the rocker member are angularly inclined relative to one another and disposed on either side of the longitudinal axis of the cylinder, the force which is transmitted by the piston to the connecting rods via the rocker member when combustion takes place is divided into two components through the arms of the rocker member. Although the force transmitted to the connecting rod which connected to the crank-arm having the angular advance is greater than that transmitted to the other connecting rod, it would be highly desirable to further increase the force transmitted to the former connecting rod for greater efficiency of the engine.